Cellular wireless is an increasingly popular means of personal communication in the modern world. In a cellular wireless network, a coverage area is divided into a number of sectors defined by radiation patterns from base stations. A mobile station, such as a cellular telephone, personal digital assistant (PDA), cellular modem, or other such device, may then communicate with the base station via a radio frequency air interface. In turn, each base station is typically coupled with other access equipment, such as a gateway or switch, to provide connectivity with a transport network such as the public switched telephone network (PSTN) or the Internet. A person using a mobile station can thereby communicate over the transport network from virtually any place inside the cellular coverage area.
An important feature of contemporary cellular wireless networks is an ability to locate the geographical position of a mobile station.
Such a feature was initially developed to assist emergency services in locating a mobile station. For example, in the United States, the Federal Communications Commission (FCC) has mandated the implementation of “Enhanced 911” (E911) services, which includes a requirement for cellular wireless carriers to report mobile station location to a public safety access point (PSAP) when connecting a call from a mobile station to the PSAP. In particular, when connecting an emergency services call, a wireless carrier may determine the location of the calling mobile station and provide an indication of that location to the called PSAP. Emergency service personnel can then be dispatched to the mobile station's location, to assist the caller.
The availability of location information to support E911 services has given rise to the development of many other location-based services as well. For instance, given the location of a mobile station, a location-based service provider (e.g., a cellular wireless carrier or third party) can provide the mobile station user with a weather or traffic report relevant for the user's location. As another example, a location-based service provider can report a list of services or establishments (e.g., restaurants, parks, theatres, etc.) in the user's vicinity. As still another example, a location-based service provider can provide a mobile station user with a map of the user's location or with directions for travel between the user's location and another location. And as yet another example, knowing that a mobile station is operating in a particular location, a location-based service provider can send the mobile station a location-based message, such as an advertisement or coupon for a nearby establishment. Other location-based services exist currently or will be developed in the future as well.
In many cases, a wireless carrier can identify the location of a mobile station based on the location of coverage provided by the sector in which the mobile station is located. In particular, a carrier usually maintains in a home location register (HLR) or other profile store an indication of the sector in which each active mobile station is operating, and a carrier also maintains or provides a set of “base station almanac” (BSA) data that specifies the geographic location of each sector in the carrier's market. Thus, when a carrier needs to determine the mobile station's location, the carrier can refer to the profile store in order to identify the sector in which the mobile station is operating, and the carrier may then refer to the BSA data to determine the geographic location of that sector. The carrier may then use that sector location as an approximation of the mobile station's location, to facilitate providing a location-based service.
For a typical sector, the BSA data may define the location of the sector by specifying various attributes such as (i) base station location, (ii) azimuth, (iii) beamwidth, and (iv) centroid. FIG. 1 helps explain these attributes. As shown in FIG. 1, a sector 12 emanates from a base station 14, which is at a location typically defined by latitude and longitude coordinates. Based on the antenna configuration at the base station, sector 12 emanates with an azimuth 16, which represents an angle measured from a reference direction such as due North for instance. As shown, the sector 12 defines a coverage angle that is substantially centered about the azimuth 16 and provides a beamwidth 18 extending in a polar direction from an initial line 20 to a terminal line 22. Based on signal strength or other data regarding the sector, a centroid 24 of the sector may then be defined as a point on the azimuth, representing approximately the center of the area of wireless coverage provided by the sector. This type of BSA data can be established by engineering analysis and input at the time base station 14 is installed or configured and can be updated from time to time.
In practice, then, when connecting an emergency services call from a mobile station, the mobile station's carrier may determine the sector in which the mobile station is operating and may then give the PSAP an indication of (i) the geographic position or street address of the serving base station tower and (ii) the azimuth at which the sector extends from the base station tower. Emergency service personnel can then work to locate the caller in that general direction from the base station tower. Further, the carrier may give the PSAP an indication of the centroid of the sector, determined based on the base station location, azimuth, beamwidth, and perhaps signal strength measurements. Emergency service personnel can then work to locate the caller around that centroid position, taking into consideration the base station location and the beamwidth of the sector.
When sector information is used as a basis to describe the location of a mobile station, the description is inherently vague, because it is not immediately clear where in the sector the mobile station is actually located. For emergency services, this presents a problem, as emergency service personnel may have trouble locating a mobile caller within a given sector. Similarly, some other location-based services may depend on knowing more precisely where a given mobile station is located, so the commercial value of those services may diminish due to the imprecision of the location information.
This can be even more of a problem in a “split sector” scenario. A “split sector” is a sector that is defined by two or more (typically two) separate coverage areas that emanate from a given base station but that are considered in combination to represent a single sector (typically sharing common coding and a common sector identification). FIG. 2 shows an example of this scenario, where a base station 30 radiates to define a sector made up of two separate coverage areas 32 and 34.
Typically, the separate coverage areas that cooperatively define a split sector will not overlap at all. However, it is possible that the coverage areas could overlap to some extent. In the usual case, the separate coverage areas will be defined by radiation from respective antenna elements of the base station, so that each coverage area has a respective azimuth and a respective beamwidth. In particular, each coverage area may be considered to define a respective coverage angle having a vertex at the base station and having an initial side and a terminal side. Each coverage area azimuth may then be the direction (e.g., vector) bisecting the angle of coverage, i.e., midway between the initial side of the coverage angle and the terminal side of the coverage angle. And each coverage area beamwidth may be the angle between the initial side of the coverage angle and the terminal side of the coverage angle; in other words, the coverage area beamwidth may equal the coverage angle. Thus, in FIG. 2, coverage area 32 has an initial side 36, a terminal side 38, a beamwidth 40, and an azimuth 42, and coverage area 34 has an initial side 44, a terminal side 46, a beamwidth 48, and an azimuth 50.
It is understood that the polar direction for designating the terminal side of an angle by comparison with the initial side of the angle is a matter of design choice. Typically, the terminal side is located counterclockwise from the initial side. However, a clockwise direction could be used instead if desired, provided that the same convention is used when referring to each coverage area of the split sector. Further, in terms of designating the angle of azimuth for a given coverage area, the reference direction (zero degrees) can be considered to be due North, or can be designated in another manner, again provided that the reference direction is used consistently. Still further, it is understood that the coverage areas being discussed here are the main lobes of the antenna radiation patterns, rather than side lobes or the like. And although the coverage areas of the split sector are shown as about the same size, it is understood that they can be different sizes then each other.
With a split sector scenario like this, an issue is how to designate the location of coverage of the sector, in the BSA data for instance. This is a particular concern where the BSA data should have just one record per sector, with the record specifying (i) base station location, (ii) a single azimuth, and (iii) a single beamwidth, among possibly other data such as a centroid location. Such a constraint on the BSA data may result from the design of position determining equipment (PDE) that makes use of the BSA data. In particular, the position determining equipment may be designed to look for a single BSA record per sector and may not be able to handle two separate records (e.g., two separate azimuths, two separate beamwidths, and two separate centroids) for a single sector. Thus, with the arrangement of FIG. 2, it may be necessary to represent the split sector in the BSA record as a single sector, such as with a single azimuth, a single beamwidth.